The invention relates to heterocyclic organometallic compounds containing aluminum, gallium or indium as metals, and to the use thereof for the production of thin films or expitaxial layers by gas phase deposition.
The deposition of such layers either from pure elements of the third group or from combinations with other elements, such as, for example, gallium arsenide, indium phosphide or gallium phosphide, can be used to produce electrical, dielectric, electronic, optical and optoelectronic structural and circuit elements, compound semiconductors and lasers. The deposition of these layers takes place from the gas phase.
The properties of these films depend on the deposition conditions and the chemical composition of the film deposited.
All the known methods such as the metal-organic chemical vapor deposition (MOCVD) method, the metal-organic molecular beam epitaxy (MOMBE) method, the photo-metal-organic vapor phase (photo-MOVP) method in which the substances are decomposed by the UV irradiation, the laser-chemical vapor deposition (laser-CVD) method or the metal-organic magnetron sputtering (MOMS) method, are suitable for deposition from the gas phase. The advantages over other methods are a controllable layer growth, a precise doping control and also simple handling and production-friendliness resulting from the normal-pressure or low-pressure conditions. Such other methods which deposit from the liquid phase are, e.g., electrochemical deposition or high vacuum methods such as electron sputtering.
In the MOCVD method, organometallic compounds are used which decompose to deposit the metal at a temperature below 1100.degree. C. Typical apparatuses currently used for MOCVD comprise a "bubbler" having a feed for the organometallic component, a reaction chamber which contains the substrate to be coated, and also a carrier gas source which should be inert towards the organometallic compound. The "bubbler" is kept at a constant, relatively low temperature which is preferably above the melting point of the organometallic compound, but far below the decomposition temperature. The reaction chamber or decomposition chamber is preferably at a very much higher temperature, below 1100.degree. C, at which the organometallic compound decomposes completely and the metal is deposited. The organometallic compound is converted to the vapour state by the carrier gas and is passed through a lock into the decomposition chamber together with the carrier gas. The mass flow rate of the vapor can readily be controlled, and a controlled growth of the thin layers is consequently also possible.
Hitherto, metal alkyls such as, for example, trimethyl gallium, trimethyl aluminium or trimethyl indium, have mainly been used for gas phase deposition. These compounds are, however, extremely sensitive to air, spontaneously ignitible and in some cases decomposable even at room temperature. Elaborate safety measures are therefore necessary for the production, transportation, storage and application of these compounds. A few, somewhat more stable, adducts of the metal alkyls with Lewis bases such as, for example, trimethylamine and triphenylphosphine, are also known (described, for example, in GB 2,123,422, EP-A 108,469 or EP-A 176,537), but these are only suitable to a limited extent for gas phase deposition owing to their low vapour pressure.
Organometallic compounds suitable for the MOCVD technique are known from German Offenlegungsschrift 3,631,469. However, the compounds described therein do not contain any compounds in which the element of the IIIrd and Vth group of the periodic system, that is to say the acceptor atom and the donor atom, are covalently linked.
It was therefore the object of the present invention to find organometallic compounds which are easy to handle and are stable at room temperature and which have a sufficiently high vapour pressure to enable them to be decomposed from the gas phase, that is to say are suitable for the various methods of gas phase deposition.
It has now been found that organometallic compounds containing the acceptor atom of the IIIrd group and two donor atoms of the Vth group in a cyclic arrangement in which either one of the donor atoms is covalently linked and the other is linked to the acceptor atom via a donor-acceptor interaction or both donor atoms are linked covalently to the acceptor atom, are outstandingly suitable for gas phase deposition.
Compared with the organometallic compounds hitherto used and known, these heterocyclic organometallic compounds have the decisive advantage that an additional intramolecular stabilization takes place by electron transfer from the donor atoms to the electron-deficient acceptor atom. The presence of at least two donor atoms thus increases the stability.
Surprisingly, in spite of this increased stability, the compounds according to the invention have sufficiently high vapour pressures to enable them to be decomposed from the gas phase.